The calculator assumes that, since you are using a refractometer, you are measuring in Brix. If you put in the pre-fermentation brix reading, it will give you the OG in gravity. Then, if you enter the terminal brix reading, it will give you the FG in gravity, corrected for alcohol content.

If you read your gravities with an hydrometer, you can enter the values in SG, but it will give you the Brix reading you would have had if you had used a refractometer.

I believe the Wort Calibration value is fixed, and should be 1.04.

When I enter 1.050 into the OG field, I get 12.67 original brix. Then, when I enter 1.021 into the Terminal Gravity field, I get 8.4 terminal brix.

My refractometer has a dual scale so I'll take the reading in Brix from now on....also don't know why my Wort calibration allowed me to change it to 1.050 if it's supposed to be fixed, but anyway I've changed it back now.

Now I'm getting the proper corrected value when I type my original readings in.